The vestibular (balance system processes multisensory information, including that of labyrinthine, visual, and proprioceptive origin, and transforms it into appropriate motor patterns. This transformation involves both spatial and temporal processing of head angular velocity and linear acceleration (including head orientation relative to gravity). A natural spatial coordinate frame for processing head rotations is the planes of the semicircular canals; indeed, the three-neuron arc of the vestibuloocular reflex largely involves pairing of canals and extraocular muscles. The central theme of this application is that three-dimensional linear accelerations also have a natural coordinate frame defined by two planes: the mirror symmetry plane of the body (sagittal plan) and the horizontal plane (defined as the plane that is normally earth-horizontal with the animal standing). These planes correspond tot he planes of the otolith organs, the (sagittal) sacculus and (horizontal) utriculus. Most movements (both of cat and man) take place either within the sagittal plane (bending, jumping, locomotion), within the horizontal plane (turning, locomotion), or involve small tilts from the horizontal plane (to maintain upright posture). Using controlled vestibular stimulation and recordings from characterized single neurons in the cat vestibular nuclei, we will determine the three-dimensional accelerations to which this neuronal population is spatially tuned; we predict that these vectors will largely lie in the horizontal and sagittal planes. Such an organizational principle would simplify motor control program(s) for postural control. In previous studies examining the horizontal spatial coding within the brainstem (lateral and inferior vestibular nuclei an adjacent reticular formation), neurons responding to lateral movements were conspicuously more numerous than neurons responding to fore-aft movements. We will target the medial vestibular nucleus to test the hypothesis that spatial tuning vectors can be found that lie in the sagittal plane (stimulated by head pitch). This hypothesis is supported by observations that lesions of the medial vestibular nucleus affect vestibulo-autonomic reflexes (which respond well to linear accelerations in the sagittal plant). Temporal information about head movement is also processed by the vestibular system. Convergence has been observed between vertical canals and otolith organs; the organization of such convergence appears to follow a principle of similar spatial alignment. We will investigate convergence between horizontal canals and otolith organs, a pattern which would be important for extending the frequency response of neurons sensing head rotations about an off vertical cervical column. Converging signals from the labyrinth can exhibit differing spatial and temporal properties. The theoretical framework for the analysis of such spatial-temporal convergence (STC) will be continued to address a number of issues. An analysis of the effect of variability in the data on the estimation of response parameters will permit specification of objective criteria for identifying instances of significant STC responses. Extension of the theory to encompass linear and angular accelerations will permit modeling of realistic head movements.